1. Technical Field
The present invention relates to a sulfur-vulcanized rubber composition and a rubber product using the same.
2. Related Art
As rubber products, those are known which require a high volume resistivity (electric resistance) and a small change in physical properties (in particular, rigidity) thereof in high temperature service environments as in automobile water hoses or the like. The reason is that the automobile water hoses are undesirably electrochemically reacted with a cooling water flowing therethrough if an electric current from a vehicle body or connection components is flowed therethrough, and also the inside of an engine room is such high temperature service environments. In Patent Documents 2 and 3, measures for increasing volume resistivity by limiting a formulation ratio of carbon black to be formulated with EPDM polymer are described.
In addition, studies are recently performed, in which rubber products are thinned to achieve weight reduction and material cost reduction and also configurations of rubber products are simplified (in a case of hoses, reduction or omission of reinforcing threads, or the like) to achieve process simplification and material cost reduction. To realize such thinning and simplification in configurations, the rubber must to have a higher rigidity, and also in a case of rubber products, such as automobile water hoses, it becomes further important that rigidity thereof is not reduced in high temperature service environments.
As measures for increasing rigidity of rubber, an increased amount of reinforcing agents, such carbon black or clay, addition of resins (thermoplastic or thermosetting), and addition of short fibers are common. However, they exhibit a high rigidity at a room temperature, but the rigidity are decreased at an elevated temperature, and accordingly, it is difficult to be applied to products, of which service temperature is high. In addition, if rigidity at an elevated temperature is ensured in anticipation of temperature dependence, the following problems are occurred.                If the reinforcing agent is increased, viscosity is increased and thus workability, such as extrusion ability, becomes poor.        If electrical conductive carbon black as the reinforcing agent is increased, volume resistivity of rubber products is decreased and thus applications of the rubber products are limited.        If electrical insulating fillers, such as clay, as the reinforcing agent are increased, temperature dependence of rigidity is increased relative to a case of carbon black, and thus it is difficult to obtain rigidity at an elevated temperature.        If resins are added, the rubber products are further hardened at a room temperature, thus handing ability thereof becomes poor.        
Therefore, the present applicant has previously proposed a rubber material, in which a combination of a co-crosslinking agent made of a metal salt and a peroxide-based vulcanizing agent is applied to a rubber, such as EPDM, and formulation ratios thereof are set at predetermined amounts, thereby reducing a change in rigidity at an elevated temperature (see Patent Document 1). However, realization of such properties in sulfur vulcanization system has been desired in terms of easiness of fabrication.
[Patent Document 1] JP-A-2011-219509
[Patent Document 2] JP-A-6-210798
[Patent Document 3] JP-A-10-180941
Accordingly, an object of the present invention is to obtain a rubber composition and a rubber product, which are sulfur vulcanization system to be easily fabricated and have a higher volume resistivity and a smaller change in rigidity at an elevated temperature, and obtained therefrom.
The inventors has found that, as a result of examination on so-called white fillers having an electrical insulation property, a difference in particle shapes of white fillers is influenced on the temperature dependence of rigidity, and more specifically, amorphous or spindle-shaped filler can suppress the temperature dependence of rigidity as compared to plate-shaped or needle-shaped filler, and accordingly has led to the present invention. The reason is believed that particle shapes of plate-shaped or needle-shaped filler are changed by following a viscous movement of polymer at an elevated temperature, but particle shapes of amorphous or spindle-shaped filler are kept without following the viscous movement of polymer even at an elevated temperature.